Manufacture of carbon nanotubes (hereinafter frequently referred to as “CNTs”) by a chemical vapor deposition (hereinafter frequently referred to as “CVD”) method or the like sometimes causes carbonaceous by-products (hereinafter frequently referred to as “carbon contaminants”), composed mainly of carbon contained in a source gas, to adhere to the inside of a manufacturing furnace. Such carbon contaminants can be removed by the methods described in JP2003146633A (PTL 1), JP2006315889A (PTL2), and JP2012250862A (PTL3), which are known in the art.
PTL 1 describes a method for gasifying carbon contaminants by supplying steam into a reaction tube used to manufacture CNTs. PTL 2 describes a method for discharging carbon contaminants in the form of carbon monoxide gas by introducing carbon dioxide into a reaction furnace used to manufacture carbon fibers and reacting the carbon dioxide with carbon contaminants. PTL 3 describes a method for removing carbon contaminants inside a growth furnace by using a cleaning gas that contains oxygen atoms.
JP2007131936A (PTL 4) describes a method for removing carbon in the form of soot or the like, which has been impregnated into the heat insulating material in a carburizing furnace, by injecting air to cause burnout.